Silane-grafted polymers are frequently used in applications requiring good temperature resistance. Prior to being exposed to moisture, silane-grafted polymers are melt-processable. However, following exposure to moisture (usually in the presence of a tin catalyst), a fully crosslinked article with excellent temperature resistance can be obtained. Silane-grafting can also be used to improve adhesion/compatibility between polyolefins and polar substrates like glass, aluminum or polyesters.
The term "grafting" as used herein, refers to insertion of one of the nitrogens from the azide group on the azidosilane molecule into a carbon-hydrogen bond on the polymer chain. A "grafting agent" is an azidosilane that is capable of reacting with a polymer chain to insert such a nitrogen into a carbon-hydrogen bond on the polymer chain.
Silane modification of a polyolefin is usually achieved via peroxide grafting of vinyltrimethoxysilanc (VTMOS) onto the polyolefin backbone. Unfortunately, this approach has limited success with polymers which degrade in the presence of free radicals (e.g., polypropylene, polystyrene, ethylene/styrene interpolymers (ESI)). Furthermore, even for polymers like polyethylene which are successfully grafted with VTMOS, grafting is usually limited to less than 2% VTMOS because of premature gelling of the polyethylene in the presence of the intermediate radical species. This premature gelling is due to the polymer chains forming radicals that can crosslink with other polymer chains as well as graft with the VTMOS.
An interpolymer is a polymer produced from at least two different monomers. As such, interpolymers include, inter alia, copolymers (two monomers) and terpolymers (three monomers).
As used here, the term "crosslinking" refers to forming a bridge between two or more polymer chains, and/or two or more positions on a single polymer chain. The bridge is formed by primary bonds between an atom or molecule (the "crosslinking agent") and atoms attached to the polymer chain. Crosslinking thus differs from grafting in that crosslinking requires that the crosslinking agent have multiple sites that can react with the polymer chains while the grafting agent can have only one such site.
Molecular weight distribution (MWD), or polydispersity, is a well known variable in polymers. The molecular weight distribution, sometimes described as the ratio of weight average molecular weight (M.sub.w) to number average molecular weight (M.sub.n) (i.e. M.sub.w /M.sub.n) can be measured directly, e.g., by gel permeation chromatography techniques, or more routinely, by measuring I.sub.10 /I.sub.2 ratio, as described in ASTM D-1238. For linear polyolefins, especially linear polyethylene, it is well known that as M.sub.w /M.sub.n. increases I.sub.10 /I.sub.2, also increases.
Traditional heterogeneous linear low density polyethylene polymers (LLDPE) or linear high density polyethylene polymers typically are made using Ziegler Natta polymerization processes (e.g., U.S. Pat. No. 4,076,698 (Anderson et al.), the disclosure of which is incorporated herein by reference), sometimes called heterogeneous polymers. For convenience, low density polyethylene will be abbreviated LDPE and high density polyethylene will be abbreviated HDPE. The Ziegler Natta polymerization process, by its catalytic nature, makes polymers which are heterogeneous, i.e., the polymer has severai different types of branching within the same polymer composition as a result of numerous metal atom catalytic sites. In addition, the heterogeneous polymers produced in the Ziegler Natta process also have broad molecular weight distributions (M.sub.w /M.sub.n); as the M.sub.w /M.sub.n increases, the I.sub.10 /I.sub.2 ratio concurrently increases.
The term "homogeneous polymers" refers to polymers made using uniform branching distribution polymerization processes. Such uniformly branched or homogeneous polymers include those linear homogeneous polymers made as described in U.S. Pat. No. 3,645,992 (Elston), the disclosure of which is incorporated herein by reference, and those linear homogeneous polymers made using so-called single site catalysts in a batch reactor having relatively high olefin concentrations (as described in U.S. Pat. No. 5,026,798 (Canich) or in U.S. Pat. No. 5,055,438 (Canich), the disclosures of which are incorporated herein by reference) or those made using constrained geometry catalysts in a batch reactor also having relatively high olefin concentrations (as described in U.S. Pat. No. 5,064,802 (Stevens et al.), the disclosure of which is incorporated herein by reference, or in EPA 0 416 815 A2 (Stevens et al.)). The uniformly branched/homogeneous polymers are those polymers in which the comonomer is randomly distributed within a given interpolymer molecule and wherein substantially all of the interpolymer molecules have the same ethylene/comonomer ratio within that interpolymer, but the linear version of these polymers have an absence of long chain branching, as, for example, Exxon Chemical has taught in their February 1992 Tappi Journal paper.
The SCBDI (Short Chain Branch Distribution Index) or CDBI (Composition Distribution Branch Index) is defined as the weight percent of the polymer molecules having a comonomer content within 50 percent of the median total molar comonomer content. The CDBI of a polymer is readily calculated from data obtained from techniques known in the art, such as, for example, temperature rising elution fractionation (abbreviated herein as "TREF") as described, for example, in Wild et al Journal of Polymer Science, Poly. Phys Ed., Vol. 20, p. 441 (1982), or as described in U.S. Pat. No. 4,798,081, U.S. Pat. No. 5,008,204, U.S. Pat. No. 5,246,783, WO 93/04486, the disclosures of each of which is incorporated herein by reference. The SCBDI or CDBI for the homogeneously branched linear or homogeneously branched substantially linear olefin polymers of the present invention is greater than about 50 percent, preferably greater than about 60 percent, and especially greater than about 80 percent.
Homogeneous polymers have a homogeneous branching distribution and typically only a single melting peak (determined using differential scanning calorimetry (DSC) using a second heat and a scan rate of 10C/minute from -40.degree. C. to 160.degree. C.), as opposed to traditional Ziegler Natta polymerized heterogeneous linear ethylene/.alpha.-olefin copolymers which have two or more melting peaks (determined using differential scanning calorimetry (DSC)).
The term "substantially linear" means that the polymer has long chain branching and that the polymer backbone is substituted with 0.01 long chain branches/1000 total carbons to 3 long chain branches/1000 total carbons, more preferably from 0.01 long chain branches/1000 total carbons to 1 long chain branches/1000 total carbons, and especially from 0.05 long chain branches/ 1000 total carbons to 1 long chain branches/1000 total carbons. The substantially linear ethylene polymers and interpolymers arc described in U.S. Pat. No. 5,272,236 and U.S. Pat. No. 5,278,272, the disclosures of which are incorporated herein by reference.
As used herein, the term "ethylene polymer" means a homopolymer made from a single ethylene or .alpha.-olefin monomer.
Long chain branching for the substantially linear ethylene polymers is defined herein as a chain length of at least 6 carbons, above which the length cannot be distinguished using .sup.13 C nuclear magnetic resonance spectroscopy. The long chain branch of the substantially linear ethylene polymers is, of course, at least one carbon longer than two carbons less than the total length of the comonomer copolymerized with ethylene. For example, in an ethylene/1-octene substantially linear polymer, the long chain branch will be at least seven carbons in length; however as a practical matter, the long chain branch has to be longer than the side chain resulting from incorporation of comonomer. For substantially linear ethylene/alpha-olefin copolymers, the long chain branch is also itself homogeneously branched, as is the backbone to which the branch is attached.